1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for measuring the carrier to interference ratio in a time division multiple access cellular network, and specifically to providing an accurate field measurement of the carrier to interference ratio on the reverse-link.
2. Background and Objects of the Present Invention
Mobile communications, especially cellular radio, is one of the fastest growing and most demanding telecommunications applications ever. Today it accommodates a large and continuously increasing percentage of all new telephone subscriptions around the world with the increasing service requirements. Cellular networks have evolved into two different networks within Time Division Multiple Access (TDMA) technology. The European cellular network uses the Global System for Mobile Communication (GSM) standard as the digital cellular system. In the United States, cellular networks have traditionally been primarily analog, but recent advances have been incorporating digital systems within the analog networks. One such North American cellular network is the D-AMPS network, which is described hereinbelow.
With reference now to FIG. 1 of the drawings, there is illustrated a D-AMPS Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Services Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18.
Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless. The MSC 14 is in communication with a Base Station (BS) 24. The BS 24 is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The radio interface between the BS 24 and the MS 20 utilizes Time Division Multiple Access (TDMA) to transmit information between the BS 24 and the MS 20, with one TDMA frame per carrier frequency. Each frame consists of six timeslots or physical channels. Depending upon the kind of information sent, different types of logical channels can be mapped onto the physical channels. For example, speech is sent on the logical channel, "Traffic Channel" (TCH), and signaling information is sent on the logical channel, "Control Channel" (CCH).
Currently, speech and data are transmitted from the BS 24 to the MS 20 on a forward-link channel 30 and from the MS 20 to the BS 24 on a reverse-link channel 32. Interference on either the forward-link or reverse-link channel can significantly reduce the quality of the signal transmitted on these channels. At present, there are two types of interference: co-channel interference and adjacent channel interference. Co-channel interference is the interference caused by the usage of the same frequency within two different clusters (not shown) of cells 22. Adjacent channel interference is caused by the usage of adjacent frequencies between adjacent cells 22 within the same cluster or within two different clusters.
The carrier-to-interference (co-channel or adjacent-channel) (C/I) ratio is one of the most important radio network performance criteria in evaluating a wireless communication system, including, but not limited to the GSM network, the new Personal Communications System (PCS) network, the D-AMPS network, and the AMPS network. In order to reduce interference within the cellular system 10, both co-channel and adjacent channel interference must be minimized. Therefore, by increasing the ratio, e.g., by reducing the interference with respect to the carrier (level) of the desired signal, the co-channel or adjacent channel interference can be reduced and the signal quality received by MSs 20 within the cell 22 can be improved.
The EIA/TIA IS-136 standards specify the minimum performance requirements of the carrier-to-co-channel interference and the carrier-to-adjacent channel interference for a BS 24. In order for vendors to ensure that their products meet C/I minimum performance requirements, the reverse-link C/I performance must be measured. These C/I field performance measurements are used by many operators as one of the criteria in comparing different vendors' products and evaluating the quality of purchased products. Furthermore, an accurate field knowledge of the C/I performance for the system is important for designing a cellular network and optimizing an existing network. However, at present, there is no accurate technique of C/I field measurement available.
It is, therefore, an object of the present invention to provide a system and method for obtaining an accurate Carrier-to-Interference field measurement.